Method for purification of lens gases used in photolithography

ABSTRACT

A method and composition for the removal of contaminants in a gas stream used in the contamination sensitive processes of photolithography and metrology are described. The synergistic effect of a combination of an electropositive metal component, a high silica zeolite, and a late transition metal compound effects removal or reduction of the contaminates in the gas which interfere with light transmittance to the ppb or ppt levels necessary for the gas to be suitable for these uses. The removal of neutral polar molecules, neutral polar aprotic molecules, protic and aprotic alkaline molecules, acidic polar species, and neutral non-polar aprotic molecules is accomplished with the claimed composition. Depending on the type of contaminant, the composition components are each varied from 10 to 80 parts by volume, with the total composition limited to 100 parts by volume.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation application of a copending U.S.patent application with Ser. No. 10/692,018 filed Oct. 23, 2003, whichis a continuation application of a U.S. patent application with Ser. No.10/078,716 filed Feb. 19, 2002 which issued as U.S. Pat. No. 6,645,898on Nov. 11, 2003, which is a divisional patent application of a U.S.patent application with Ser. No. 09/824,382 filed on Apr. 2, 2001 whichissued as U.S. Pat. No. 6,391,090 on May 21, 2002. All of these patentsand patent applications are hereby incorporated herein by reference intheir entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention herein relates to the purification of gas. Moreparticularly, it pertains to the purification of gases which are used inproduction of contamination-sensitive products.

2. Description of the Background

The circuitry on semiconductor chips is normally formed byphotolithography. Each layer of the chip has a photoresist mask whichdefines the circuitry for that layer. Ultraviolet laser produced lightis used to expose the photoresist and create the image of the circuitryon the chip layer.

The degree of definition of the circuit lines on the chip is a functionof the wavelength of the laser light. As circuitry has become morecomplex, more and more circuit lines are being crowded into each unitarea on the semiconductor layer. Not only does this mean that the linesthemselves must be narrower, but it also means that their definitionmust be more precise to avoid either interference and short circuitingwith neighboring lines or breaks (opens) in an individual line. Therelationship between the ultraviolet light wavelength and the width ofthe circuit lines is direct: narrower circuit lines require shorterultraviolet light wavelengths. Conversely, the shorter wavelengths ofultraviolet light have higher energies. Ultraviolet is generally definedas wavelengths between 400 nanometers (nm) and about 10 nm. Otherregions of interest are Near Ultraviolet (NUV), which is approximately400 to 300 nm; Deep Ultraviolet (DUV), which is approximately 300 to 100nm; and Extreme Ultraviolet (EUV), which is approximately ≦100 nm.Currently commercial semiconductor production processes operate with 248nm (KrF) and 193 nm (ArF) ultraviolet light wavelengths which allowproduction of circuit lines with widths of approximately 0.15micrometers (Am). The industry currently projects that the line widthsof 0.10 μm and smaller will become the standard shortly, which willrequire photolithography with ultraviolet light wavelengths of 157 nm.

Photolithography of semiconductor wafers is conducted in closed chamberswith the laser lens and the target wafer being surrounded by a gaseousatmosphere. The gases used in process environments vary from compresseddry air (CDA) to inert gases. Common examples are CDA, He, N₂, and He/O₂mixtures. Different manufacturers, processes, wavelengths of light, andother requirements favor different gases. As the field advances to lowerwavelengths of light, i.e. more energetic light, inert gases will likelybecome dominant, especially N₂ and He.

It is normal, however, for the gas itself to be contaminated with smallamounts of reactive gases or vapors or particulate materials. All thesecontaminants can and do affect the production process in various ways.Molecular contaminants with absorbances in the UV range reduce theoptical transmittance of the lithography tool. Residues, deposits, andcondensates form on the optical components of the lithography tool whichreduce light transmittance. The photoacids generated by photoresistsduring the lithography process are sensitive to quenching by molecularcontaminants, especially alkaline contaminants like ammonia. In sub-248nm lithography, these contaminants can be found in some or all of thecomponents of the lithography tool.

A limited number of decontamination processes and products have beenused in the past to produce “lens gases” of acceptable levels of purity.Most notable of these are carbon beds and particulate filters. As therequired circuit line width and therefore the ultraviolet lightwavelengths have decreased, however, decontamination processes whichwere once sufficient have become unacceptable because the degree ofresidual contamination has been found to be too high under the newer,more stringent requirements.

Metrology is the field within the semiconductor industry that isresponsible for testing the wafer at various stages in the fabricationprocess. Wafers are tested by metrology devices throughout theirmanufacturing to ensure compliance and eliminate defective wafers.Metrology limits must always exceed the resolution limits of thelithographic processes. As device dimensions shrink metrology limitsmust follow suit, which will lead to greater contaminationsusceptibility in the metrology environment.

Two technologies currently dominate the metrology market: opticalmetrology and electron beam metrology. Purification of the gases in themetrology environment is crucial to the accuracy of metrology.

For example, optical metrology devices may be within or outside thelithography tool itself. When the device is in the tool the metrologyoptics and laser beam become an integral part of the lithography tool.These systems are used for continuous monitoring of masks and reticles.The necessity of gas purification is the same as for the lithographytool environment.

In-line optical measurements are found outside the lithographyenvironment. For example, ellipsometry tools are commonly used tomeasure thin-film uniformity after spin-on resist application. Thephotoresists in ellipsometers and interferometers are susceptible tomolecular contamination from the environment.

CD-SEM is used as an inline metrology tool that has a high resolution.Although CD-SEM operation is conducted in a high vacuum chamber, thechamber is periodically opened to atmospheric contaminants, therefore,the purity of the purge gas is crucial in ensuring long-term toolstability. Additionally, the cassette(s) may be contained in achemically purified environment to minimize their contamination.Furthermore, SEM reticle inspection systems may require purification tominimize damage to the reticle from molecular contaminants, especiallywhen exotic materials become essential in future technology modes.Reticles may adsorb contaminants then desorb under process conditions.

The electron beam in CD-SEM produces charge contaminants in wafers.These occur when electrons are ejected from or adsorbed by the wafersurface. Therefore, another metrology tool must non-destructivelymeasure the charge contamination after SEM and must also be contained ina decontaminated environment.

There are two kinds of contaminants that need to be considered in anypurification process: those that are in the lens gas originally andthose that get injected into the gas stream during the purificationprocess. The contaminants commonly present in the lithographyenvironment are water (H₂O), hydrocarbons (H_(x) C_(y)), oxides ofnitrogen (NO_(x)), oxides of sulfur (SO_(x)), ammonia (NH₃), organicamines (R₃N), metals (M⁺), halogenated and sulfonated hydrocarbons (RX,X═F, Cl, Br, I, SO₃), siloxanes (Si_(x)H_(y)), alcohols (ROH), sulfides(R₂S), and halogen hydrides (HX, where X═F, Cl, Br or I), among others.Certain groups of contaminants present different problems inphotolithography.

-   -   i. The group of neutral polar protic molecules—e.g., H₂O and        ROH—absorb radiation in the sub-248 nm range, condense easily on        surfaces, and stabilize other polar contaminants by forming        solvation complexes.    -   ii. The group of neutral polar aprotic molecules—e.g., NO_(x),        SO_(x), R₂S, and RX—absorb radiation in the sub-248 nm range and        react with other contaminants to form harmful products. For        example, the formation of ammonium sulfate residues on optical        surfaces, known as optical hazing.    -   iii. The group of alkaline molecules, both protic and        aprotic—e.g., amines, including ammonia—absorb in the sub-248 nm        range; react with other contaminants; and quench the photoacid        generated in the resist film. This quenching results in an        undesirable T-shaped resist profile, known as T-topping. iv. The        group of acidic polar species, both Lewis (M⁺) and Bronsted        (HX), absorb in the sub-248 nm range and react with other        contaminants, mainly from Group (i), to form harmful acidic        products. Bronsted acids on metal lead to loss of process        control because the photoresists are based on specific acid        content based on a timed release agent. Additional acid        interferes with the control.    -   v. The group of neutral non-polar aprotic molecules—e.g.,        hydrocarbons and siloxanes—absorb in the sub-248 nm range,        condense easily on surfaces, and are ubiquitous in the cleanroom        environment. Under high energy conditions, hydrocarbons will        leave a carbonaceous residue on the lens. Siloxanes can form an        opaque layer of SiO₂ on the lens.    -   vi. The group of environmental gases—e.g., CO₂, and CO—absorb in        the sub248 nm range and are ubiquitous in the cleanroom        environment.

Contaminants also get injected into the gas stream during thepurification process commonly from the decontamination material itself.For example, upstream carbon beds can release particulate contamination.Decontamination materials are normally high surface area solid materialseither in granular form or as surface coatings on solid substrates. Theflow of the gas being decontaminated or the process of surfaceadsorption can cause minute particulates to be generated from thedecontamination>material and entrained in the gas stream. Moreover, itis well known that the mixtures and concentrations of contaminants ingas streams vary from photolithography process to process and from gasvendor to gas vendor. Therefore, operators of individualphotolithography processes must shop for different decontaminationproducts depending on their specific process. Since most decontaminationproducts have relatively narrow use limitations, in many cases a processoperator cannot find an optimum decontamination product for the specificprocess and must compromise its decontamination specifications.

To eliminate contaminants from the lithography system, some or all ofthe components of the lithography tool are enclosed in sealedcompartments. FIG. 1 demonstrates a typical lithography tool. The entiremicrolithography tool has not commonly been enclosed in its own chamber,but the trend toward improved molecular purity will require suchisolation of the environment within the tool. Since the wafer must beplaced in the laser path and removed from the tool, the environment mustbe constantly refreshed. Therefore, point-of-use purification of thelithography tool gas is necessary. In contrast, the laser is normallyenclosed in a permanently sealed compartment. The laser is often sealedat its manufacturing facility, which is usually in a separate locationfrom the tool assembly. The disadvantage to this practice is that itrequires that a decontamination composition must also function for thelifetime of the laser. Therefore, purification of laser gas both priorto and during laser assembly, as well as point-of-use purification, isnecessary.

The optics compartment is generally sealed from the rest of the toolenvironment, but it is often accessed by the operators, whereas thelaser is not. Even if the entire lithography tool is contained in apurified environment, the optics portion will be further isolated, as itis not opened to the external environment as frequently as thelithography tool. Therefore, purification of the gases contained in theoptics compartment, both at the manufacturing facility and point-of-useis necessary.

Typically, the gases present in the equipment are CDA, N₂, He or Ar,and/or He/O₂. Specifically, in the tool compartment and opticscompartment, all of these gases may be present. In the lasercompartment, N₂ and He or Ar may be present. In scanning electronmicroscopy and other metrology tools, CDA and N₂ are present. In thefuture, He or Ar may be used for these measuring tools.

Critical dimension (CD) quality and processing efficiency may alsobenefit from isolation and purification of the environments of variousadditional device uses in the semiconductor industry, e.g. bake ovensand metrology equipment. It may also become possible and beneficial toisolate the entire process of semiconductor manufacturing, from barewafers to functional chips. Therefore, purification during assembly andpoint of use throughout the semiconductor industry is necessary.

It will therefore be evident that as the photolithography technologyadvances to lower ultraviolet light wavelengths and lower widthmicrocircuitry lines the degree of decontamination of the lens gasesmust become greater, both for the removal of the original contaminantsin the gas and also for the prevention of creation and injection ofparticulate contaminants into the gas during the decontaminationprocess.

The Semiconductor Industry Association recommends that acceptable levelsfor all contaminants be in the 10-100 ppt (parts per trillion) rangeover the next few years. Problems have arisen in reaching that goal.Contaminant levels and decontamination requirements vary widely anddepend on the gas, process, wavelength of light, and CD requirements.Other factors that increase the difficulty of reaching this goalinclude: the wide range of gaseous environments in different tools anddifferent compartments within the tools, the wide range of possiblecontaminants and wide range of possible concentrations, the presence ofnon-atmospheric contaminants in combination with atmosphericcontaminants, and the interference of certain contaminants with theattempted removal of other contaminants.

A single formulation capable of accomplishing sub-ppb levels for allcontaminants does not presently exist. Some current purificationtechnologies require energetic activation, either with heat orelectrical stimulation, which may adversely affect the contaminationlevels by outgassing and by-product formation. Some purificationtechnologies release particulate components that become entrained in thegas stream, especially at high pressure. Further, some purificationtechnologies require periodic regeneration, replacement, and/oractivation during the serviceable lifetime of the lithography tool. Thiscauses an interruption in production and may increase the likelihood ofoutgassing, by-product formation, and/or particulate entraining.

SUMMARY OF THE INVENTION

The invention herein overcomes the deficiencies of the priordecontaminating agents and provides single composition decontaminationproducts which can be used for point-of-use decontamination of a widevariety of contaminant gases in lens gas streams, can be preciselyformulated for individual specific decontamination projects, and laststhe entire serviceable lifetime of the device. The invention is a classof compositions that comprise three types of materials which functiontogether to allow decontamination of lens gases down to a level ofsub-ppb (parts per billion), preferably less than 100 parts per trillion(ppt), a level previously unattainable by existing technology.Furthermore, this level of decontamination is becoming necessary in thefield of UV photolithography and metrology.

Decontamination at sub-ppb levels, preferably less than 100 ppt, isaccomplished by combining a novel contaminant adsorbing material withtwo known contaminant adsorbing materials. Synergy between the threematerials produces previously unattainable levels of purification. Thefirst material is a high surface area oxide of an electropositivetransition metal or lanthanide metal. The second material is a highsilica zeolite. The third material is an oxide of a late transitionmetal or a reduced late transition metal supported on a high surfacearea inorganic material.

The ratios of the three components may be varied according to theformula aA+bB+cC=1, where a, b, and c each is in the range of 0.1-0.8.The concentrations of the three principal components of the compositioncan be varied to enable the composition to be used for decontaminationof a wide variety of potential contaminant gases in the photolithographygas stream. The composition limits are from 10 to 80 percent by volumeof each component. Thus by selecting the appropriate ratios of thecomponents, a composition of the present invention can be tailoredprecisely to the specific mix of contaminants in the gas stream forindividual processes. The range of possible compositions is defined bythe area within the dashed line A-B-C in FIG. 2. As used herein, “inert”means that the gas is inert to the operating components of thephotolithography or metrology process and its products. The “inert”gases of the present invention include, but are not limited to, gasescommonly labeled “noble” or “inert” generally in the gas industry.

As noted, the lens gases which are of principal interest and most widelyused in the photolithography processes are nitrogen, CDA, helium, Ar,and helium/oxygen mixtures, although it is anticipated that the presentcompositions will be useful with any inert lens gas compositions,including gas compounds or mono-elemental gases which may be formulatedor specified for individual unique situations. The gases should notconflict with the absorption. For example, O₂ is not a contaminant atany wavelength other than 157 nm, since it absorbs light there.Therefore, the gas must be chosen by user with the intended UVwavelength in mind.

Specifically, the compositions of this invention comprise 10 to 80percent by volume of an electropositive metal; 10 to 80 percent byvolume of a high silica zeolite; and 10 to 80 percent by volume of alate transition metal compound, with the total being 100 percent byvolume.

The compositions are capable of removing both atmospheric andnon-atmospheric contaminants to sub-ppb levels from the purge gases usedin various environments pertaining to NUV, DUV, and EUVphotolithography. The invention removes neutral polar molecules—e.g. H₂Oand ROH; neutral polar aprotic molecules—eg. NO_(x), SO_(x), R₂S, andRX; alkaline molecules, both protic and aprotic—e.g. amines, includingammonia; acidic polar species, both Lewis (M⁺) and Bronsted (HX); andneutral non-polar aprotic molecules—e.g., hydrocarbons and siloxanes.

The invention also removes particulate matter, but it is preferred thatparticulate removal devices or systems be positioned upstream of theinventive decontamination compositions in the inert lens gas flow streamto reduce the particulate load of the gas stream to a low level. Suchparticulate removal systems and devices are readily available, and theiruse reduces the service requirements on the present compositions andmake them more available for their principal function, removal of thegaseous and vaporous contaminants from the gas stream.

The compositions of the present invention also provide the advantageousproperty that they do not themselves contribute any contaminants to thegas streams. They do not generate particulates in any significant amountnor do they release reaction products of the adsorbed gases. Thus theydo not produce any “injected” contamination into the gas stream.

The present invention operates at a variety of pressures and issufficiently stable under the operating conditions as to preventparticulates from becoming entrained in the gas stream.

The compositions of the present invention also have long service lives.They will operate effectively for five to seven years or more, whichtime span represents the normal service life of a photolithography tool.The invention does not require regeneration or energetic activationduring the entire serviceable lifetime.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a typical lithography tool,illustrating the typical compartments where decontaminates gas may bepresent.

FIG. 2 is a triangular coordinate graph representing the range ofcompositions of this invention. The hatch marks on the vertical axes areat 10 volume percent intervals, with an apex representing 100 percent ofa single component.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS

The invention is best understood by descriptions of the three principalcomponents of the present compositions: the high surface areaelectropositive metal component, the high silica zeolite component, andthe late transition metal oxide. In the descriptions and claims herein,all concentrations are in parts or percent by volume unless otherwisestated.

Synergy between the three materials produces previously unattainablelevels of purification. It is the synergy between the three materialsthat is highly effective. The first two components (electropositivemetals and high silica zeolite) are used herein to effect the removal ofcertain contaminants that interfere with the action of the thirdcomponent (late transition metal compounds). Likewise, the firstcomponent removes contaminants that potentially interfere with thesorbent capacity of the second component. Therefore, the presence of allthree of these materials is necessary to affect the desired sub-ppb,preferably less than 100 ppt, levels of decontamination. The ratiosof-the three materials are adjusted according to the formula aA+bB+cC=1,where a, b, and c each is in the range of 0.1-0.8, depending upon thespecific decontamination needs.

The Electropositive Metal Component

The electropositive metal component comprises certain electropositivemetals (Groups 3 and 4 metals, the lanthanide metals and vanadium) andtheir salts and oxides. These provide an affinity for water and with asurface area of at least about 140 m²/g and can be used very effectivelyto reduce water content in the inert lens gas streams down to the orderof 1 ppb or lower. They are also effective in removal of hydrocarbongases and carbon oxides from the lens gas stream. The metals, oxides andsalts must be compatible (i.e., nonreactive) with all gases in the lensgas stream to be dried.

The described electropositive metals, metal oxides and salts have beenfound to provide unique specific properties synergistically to thepresent compositions. Specifically they will be metals, metal oxides andmetal salts of the electropositive Group 3 metals (scandium, yttrium andlanthanum), Group 4 metals (titanium, zirconium and hafnium), vanadium,and the lanthanide metals (cerium, praseodymium, neodymium, samarium,europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium,ytterbium and lutetium). (Promethium and actinium also fall within thisgroup, but since they are unstable they are not of interest in thisinvention.). Preferred among the various materials which are usefulherein as dehydration agents are titania, zirconia, yttria and vanadia.

The metals, oxides, or salts of this component will have surface areasof at least 140 m²/g, preferably in the range of 140-1200 m²/g, commonlyin the range of 140-500 m²/g. There are currently a limited number ofcommercially available materials with this high level of surface area,and which also meet the other requirements described. It is anticipatedthat additional suitable materials will become commercially available inthe future and it is to be understood that such forthcoming materialsare considered to be within the scope of this invention.

Preferably the high surface area metal, metal oxide or saltdecontaminating agent will be a high surface area titania. However,numerous other metal oxides or salts, such as zirconia, yttria orvanadia with adequate surface areas will be also satisfactory. Thepreferred titania has a surface area in the range of 140-250 m²/g and isa product commercially available from the Engelhard Minerals andChemicals Corporation as a petrochemical catalyst for olefintransformation reactions.

It is preferred to activate the electropositive metal component prior toincorporation of the compositions into the lens gas system. Activationcan be accomplished by heating the high surface area agent in ananhydrous inert gas atmosphere (e.g., N₂ or Ar gas) at a temperature ofabout 250°-425° C. (480°-800° F.) for about 24-48 hours. The activatinggas itself must be dehydrated prior to the activation procedure.

The electropositive metal component will be present in the compositionsas 10 to 80 percent by volume. This component has a high affinity forwater, but is inert to oxygen, which is a major component in many lensgas systems. When the lens gas has a high water content, theelectropositive metal component should be in the upper end of itsconcentration range in the compositions, and conversely it can be inlower concentration when the water content of the lens gas is lower. Thesame considerations hold when there is a high contaminant content ofcarbon oxides.

Typically, the lens gases will have water contamination contents in therange of about 10-100 ppm, with some as low as 500 ppb. Water at 100 ppmis 4-5 orders of magnitude more concentrated than most non-atmosphericcontaminants (e.g. SO₂ at 5 ppb). Therefore, the effective removal ofwater is of the utmost importance. After contact with the compositionsherein, the decontaminated lens gases will have water contents nogreater than about than 1 ppb, and often as low as 50 ppt. Those skilledin the photolithography art will be readily able to use the compositionsof this invention in the manner described and obtain the level ofdecontamination required for the specific products of interest.

The High Silica Zeolite Component

The high silica zeolite component may be any of the high silica zeoliteswhich are effective to reduce hydrocarbon gas levels down to 1 ppb orlower, and in many cases down to the order of 100 ppt. Particularlypreferred zeolites are those high silica synthetic zeolites commerciallyavailable under the names Zeolite Y and ZSM-5 and their analogs.

Zeolites are a class of synthetic and natural minerals having analuminosilicate tetrahedral framework, ion-exchangable large cations,and 10%-20% loosely held water molecules which permit reversibledehydration without significant alteration in the molecular structure.They are often referred to as “molecular sieves” because of theirability to separate gaseous and liquid molecules on the basis ofmolecular size. The metal cations present are primarily sodium andcalcium, but may also include various alkali metal or alkaline elementssuch as potassium, strontium and barium. To be suitable for the presentinvention, the zeolites must have the water removed and also the aluminacontent must be reduce to a point where the silica is the predominantcomponent of the zeolite structure. Particularly preferred in thisinvention are as indicated the commercial synthetic zeolites Zeolite Yand ZSM-5.

The zeolites are a well known and widely described class of natural andsynthetic aluminosilicates. For the purposes of this invention, the term“zeolite” will mean any aluminosilicate, natural or synthetic, which hasa crystalline structure substantially equivalent to that of the mineralsclassified as zeolites. The natural zeolites have been widely describedin standard mineralogy texts for many years; particularly gooddescriptions are found in Dana, A TEXTBOOK OF MINERALOGY, pp. 640-675(4th ed. [rev'd. by Ford]: 1932); Deer et al., AN INTRODUCTION TO THEROCK FORMING MINERALS, pp. 393-402 (1966) and Kuhl et al., “MolecularSieves,” in Ruthven, ed., ENCYCLOPEDIA OF SEPARATION TECHNOLOGY, vol. 2,pp. 1339-1369 (1997).

The synthetic zeolites, which have been developed primarily for use inchemical and petroleum catalytic processes, are often referred to by theprefix word “synthetic” attached to the name of their naturalcounterparts, or, for those synthetic zeolites which do not have naturalcounterparts, by various coined names, such as Zeolite A, Zeolite X,Zeolite Y, ZSM-5, Zeolite 13X and so forth. An excellent description ofthe synthetic zeolites and their manufacture and uses as catalysts willbe found in the Kuihl et. al. reference cited above, in Elvers et al.(eds), ULLMANN'S ENCYCLOPEDIA OF INDUSTRIAL CHEMISTRY, “Zeolites”, vol.A28, pp. 475-504 (5th Comp. Rev. Edn., 1985) and in Kirk-Othmer,ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, “Molecular Sieves”, vol. 15, pp.638-669 (3rd Edn., 1978).

Most preferred are the synthetic zeolites referred to as Zeolite Y(sometimes referred to as “Zeolite NaY”) and zeolite ZSM-5. These aredescribed in detail in ULLMANN'S, supra, at p. 479 and at pp. 486 and487, respectively. Where any zeolite, including Zeolite Y or zeoliteZSM-5, might have an unduly low SiO₂/Al₂O₃ ratio upon formation, thatratio can be enhanced to an appropriate level by conventional techniquessuch as those also described in ULLMANN's at pages 489-490. Further,there have been developments in synthetic zeolite technology since thepublication of the ULLMANN reference, such that there are now widelyavailable commercial zeolites with high surface area, commonly in therange of 400-900 m²/gm.

The high silica zeolites useful in this invention will have asilica:alumina ratio of at least 90:1, preferably at least 300:1, andmore preferably at least 400:1. A preferred range is 400-2000:1,although high silica zeolites with ratios as high as 3000:1 have beenprepared and it is anticipated that the higher ratios will be preferredin specific applications. Their use is therefore contemplated in thisinvention when they become commercially available. Surface areas of thehigh silica zeolites are typically up to about 1500 m²/gm, preferably inthe range of 400-1500 m²/gm. Most commercially available high-silicazeolites are in the range of about 400-900 m²/gm. Normally the highsilica zeolites are prepared by treating the original natural orsynthetic zeolite with a reactant specific to alumina, so that thealumina content is substantially reduced without affecting the silicacontent or significantly altering the zeolite structure.

It is believed that the critical aspect which determines suitability ofa particular zeolite for the present invention is the ability to undergoalumina removal without significant alteration in the zeolite structure.For instance, the preferred materials, Zeolite Y and zeolite ZSM-5 areconsidered to be quite suitable for decontamination of a lens gasstream, both because they are good adsorbents and also because of theirstructural stability in the flowing gas stream.

For most of the high silica zeolites, including the preferred highsilica Zeolites Y and zeolite ZSM-5, it is preferred to activate thematerial prior to incorporation into the system for decontamination.Activation can be accomplished by heating the high silica zeolite in ananhydrous inert gas atmosphere (e.g., N₂ or Ar gas) at a temperature ofabout 250°-425° C. (480-800° F.) for about 24-48 hours. The activatinggas itself must be purified prior to the activation procedure.

The Late Transition Metal Oxide Component

The metal oxides useful in the claimed invention are the late transitionmetal oxides, specifically those of Groups 7-14 metals, preferably thoseof groups 10-14 metals, most preferably iron, copper, nickel or zincoxides. These metal oxides are well known for their catalyticproperties. Good adsorption properties are a key aspect of theircatalytic properties. However, under normal catalytic conditions thematerials'high affinity for certain compounds leads to the detrimentalhindering and destroying of these properties. One well-known example ofthis phenomenon is “sulfur poisoning.” In the present invention thesedetrimental adsorptive properties are exploited to remove certaincontaminants from the lens gas, specifically the non-atmosphericcontaminants.

The metal oxides are normally deposited on an inorganic substrate with ahigh surface of ≧100 m²/g. Many substrates (SiO₂, Al₂O₃, TiO₂, MgO,etc.) are suitable for this purpose and widely available. However,should the pure metal oxides become available in a high surface areaform, their direct use may be desirable in the claimed invention.Therefore, one of skill in the art would understand that this embodimentis not precluded from the claimed invention.

The metal oxides need not be in their highest oxidation states. Mixedoxides which have the desired adsorption and physical properties areincluded in this invention. Metal oxides in various oxidation states maybe present at different times in the life of the material. The oxidesmay be pretreated (e.g. 250-425° C., for 24-48 hours), but need not bereduced. This is mainly to remove H₂O, CO, and CO₂, as well as anyhydrocarbons adsorbed by substrate. While they are not normally reduced,this is not precluded from the relevant embodiments of this invention.

The late transition metal or metal compound component of the presentcomposition will be an active high surface area metal or metal oxide forremoval of various gaseous contaminants, notably non-atmosphericcontaminants such as SO_(x), NO_(x), R₂S, M⁺, and HX. Reduction ofcontaminant level is to a level ≦1 ppb, and preferably ≦100 ppt. (It isrecognized that for some photolithography processes may not haveavailable contaminant measuring equipment capable of measuringcontaminant concentration as low as 1 ppt or less. However, theavailability, or lack thereof, of appropriate measuring equipment doesnot alter the various levels of decontamination which are called for andcan be achieved in the present invention.) These contaminants are notadsorbed by the other two materials and present a considerablepurification challenge when high concentrations of atmosphericcontaminants are present. However, this material does not workeffectively without the other two materials.

It is preferred that the surface area of the metal oxide-containingmaterial be greater than 100 m²/g.

This is the first known use of oxides of late transition metals in gaspurification technology for the semiconductor industry. The purpose ofthe late transition metal oxides is to take advantage of their excellentsorption capacities for the non-atmospheric components. Additionally,the late transition metals, even in oxidized form, will have a capacityto adsorb additional atmospheric contaminants remaining in the gasstream after contact with the previous two sorbents. These includecontaminants described in the Background section as belonging to Group(i), e.g., H₂O, and Group (vi), e.g. CO₂.

As noted above, many different metals and their oxides are useful in thepresent invention, as long as they have the requisite high surface areaand maintain their structural stability (either alone, mixed or incombination with another metal oxide which has greater structuralintegrity in the presence of the gas stream). By “structural integrity”is meant that the metal oxide substrate can resist erosion or breakagein the presence of the flowing gas stream, and does not deteriorate bysuffering reduction of surface area below the minimum 100 m²/g limit.

While the level of purification available by use of the late transitionmetal oxides is new to the industry, this does not negate the importanceof the first two sorbent media described. Although these materials havebeen previously described in the gas purification art, they are usedherein to effect the removal of certain contaminants that interfere withthe action of the third component. Likewise, the first component removescontaminants that potentially interfere with the sorbent capacity of thesecond component. Therefore, the presence of all three of thesematerials is necessary to affect the desired sub-ppb, preferably lessthan-100 ppt, levels of decontamination. The amounts of each componentcan be varied to decontaminate a variety of types of contaminants.

This level was previously unattainable, particularly in high backgroundlevels of interfering components, specifically atmospheric contaminants.Using these three components, water concentration can be reduce bygreater than six orders of magnitude. Reducing contaminants to this lowlevel has not been necessary in the past, but at the new higherdefinition levels, the reduction of contaminants to the subppb level isnecessary. The composition leaves the lens gases themselves unaffectedbecause the gas stream is purified without introduction of anycontaminants or particulates into the gas stream.

One important and unique aspect of the present compositions is theirability to be tailored to specific contaminant profiles of individuallens gas streams. Thus where water content is high, the compositionratios can be set such that the electropositive metal component and/orthe high silica zeolite component are predominant, while when oxidegases or hydrogen sulfide are high, the late transition metal componentscan be more dominant in the compositions.

The Composition

The claimed composition is useful for purifying gas streams used inphotolithography, especially when the gas stream contains highconcentrations of background contaminants and low levels of thesecontaminants are necessary. The percentage, by volume, of the threecomponents is varied depending on the level and number of contaminants.It is the synergistic effects of the three components that achieves thelow levels of contaminants.

The ratios of the three components may be varied according to theformula aA+bB+cC=1, where a, b, and c each is in the range of 0.1-0.8.The concentrations of the three principal components of the compositioncan be varied to enable the composition to be used for decontaminationof a wide variety of potential contaminant gases in the photolithographygas stream. The composition limits are from 10 to 80 percent by volumeof each component. Thus by selecting the appropriate ratios of thecomponents, a composition of the present invention can be tailoredprecisely to the specific mix of contaminants in the gas stream forindividual processes.

The ratios of the three decontamination materials may be chosenaccording to the specific needs of the environment, and the specificcontaminants contained therein. This is best shown by the graph of FIG.2. All of the compositions useful herein will lie on or within the lineA-B-C. From the compositions that are defined by that graphed area theuser will select an optimum composition depending on the type(s) ofcontaminants which are to be removed. Different types of contaminantswill dictate compositions from different parts of the graphed area. Forinstance, a high concentration of Group (i) contaminants (e.g. H₂O orROH), a high concentration of Group (ii) contaminants (e.g. NO_(x),SO_(x), R₂S, or RX) and a high concentration of Group (iii) (alkalinemolecules, both protic and aprotic) contaminants would require higherratios of the electropositive material and late transition metal oxides(from left side of FIG. 2), with relatively low levels of the highsilica zeolite, from the right side of the graphed area of FIG. 2.Conversely, a high concentration of Group (iii) (alkaline molecules),Group (iv) (e.g. Lewis(M⁺) or Bronsted (HX)), Group (v) (e.g. Hcs orsiloxanes), and Group (vi) (e.g. CO or CO₂) contaminants will requirehigher ratios of the high silica zeolite and late transition metaloxide, from the lower portion of the graphed area of FIG. 2.

The following examples are given to enable those of ordinary skill inthe art to more clearly understand and to practice the invention. Theexamples should not be considered as limiting the scope of theinvention, but merely as illustrative and representative thereof. Itwill be understood by one skilled in the art that this invention isapplicable to future lithographic and metrological advances that requiregas purification.

EXAMPLE 1

A typical composition for use in a high moisture environment willcontain 70% of an electropositive metal component, 15% of a high silicazeolite, and 15% of a late transition metal compound. This compositionis indicated by data point 102 in FIG. 2.

EXAMPLE 2

A typical composition for use in an environment high in hydrocarbonsand/or siloxanes will contain 40% of an electropositive metal component,50% of a high silica zeolite, and 10% of a late transition metalcompound. This composition is indicated by data point 104 in FIG. 2.

EXAMPLE 3

A typical composition for use in an environment high in amines and/oracids will contain 40% of an electropositive metal component, 20% of ahigh silica zeolite, and 40% of a late transition metal compound. Thiscomposition is indicated by data point 106 in FIG. 2.

EXAMPLE 4

A typical composition for use in an environment high in sulfur oxides ornitrogen oxides will contain 30% of an electropositive metal component,20% of a high silica zeolite, and 50% of a late transition metalcompound. This composition is indicated by data point 108 in FIG. 2.

It will be understood, of course, that each of these examples indicatesa general range on and within the A-B-C region of FIG. 2 in which thevarious compositions are tailored to the specific type of contaminantenvironment. It will also be understood that such regions will have atransition between them and can even overlap, as suggested by datapoints 106 and 108, rather than being sharply defined or mutuallyexclusive. The same composition may in fact be quite useful for morethan one type of contaminant environment. Those skilled in the art willhave no difficulty selecting the specific composition optimum for theirparticular contaminant environments from the range of compositions inFIG. 2.

The compositions, as discussed above, can be in particulate, coating,pelletted, extruded, plate or powder form, or a mixture of these.Conveniently, regardless of form, a composition will be housed in adurable high purity container, such as a cannister, preferably formed ofa long service life metal such as stainless steel, through which thelens gas is flowed. The inlet gas will have a certain contaminant level,most of which will be removed during the gas' transit through thecannister, so that the lens gas upon exiting from the cannister willhave a much reduced contaminant load, on the order of 1 ppb or less,preferably on the order to 100 ppt. Other types of housings, containers,canisters, etc. may be used, as long as they have service livescommensurate with the service lives of the composition and the laser andare of ultra high purity. The housing, container, etc. may be a separateunit or may be incorporated into one or more units of thephotolithography system, such as the wafer production chamber.

Also as discussed above, it may be advantageous to have a stageddecontamination system, in which one or more conventional contaminantremoval units are placed upstream (preferably) or downstream of thecompositions of this invention, so that the load of individualcomponents may be reduced somewhat prior to arrival of the lens gas atthe present composition. Many such “pre-treatment” units are available,especially for dehydration and particulate removal. By lessening thecontaminant load of the lens gas prior to its contact of the presentcompositions, the present compositions operate more efficiently and havea longer service life before they become saturated or deactivated. Thoseskilled in the art will be well aware of such pretreating units and thedetails of their use.

The components can be used in a variety of different embodiments. In apreferred embodiment, the titania is in the physical form of pellets orlarge granules. One can simply pass the lens gas through the compositiondisposed in body consisting substantially or essentially of the threecomponents in pellet or granule form, or the like. The components canalso be in the form of a body of comminuted fine powders. However, sinceit is preferred to keep the system gas pressure drop below 1-5 mbar andmaintain high gas flow rates, using such powders may cause a greaterpressure drop in the gas stream, so it is preferred to used the powderedform only in high lens gas pressure systems. It is thus possible to havedifferent forms of the compositions and components for gas streams ofdifferent pressures, by using different physical sizes. With the smallerparticle or granule sizes care must be taken to minimize entrainment.

It will be evident that there are numerous embodiments of the presentinvention which are not expressly described above but which are clearlywithin the scope and spirit of the present invention. The abovedescription is therefore intended to be exemplary only, and the actualscope of the invention is to be determined from the appended claims.

1. A method of decontaminating a gas, wherein said gas comprises one ormore of neutral polar protic, neutral polar aprotic, alkaline and polaracidic contaminants, the method comprising removing said contaminantsfrom the gas by passing the gas through a body of decontaminantcomprising 10% to 80% by volume an electropositive metal component, 10%to 80% by volume of a late transition metal compound component and 10%to 80% by volume of a high silica zeolite component.
 2. The method ofclaim 1, wherein the decontaminant comprises a greater proportion of theelectropositive metal component than the high silica zeolite component.3. The method of claim 1, wherein the decontaminant comprises a smallerproportion of high silica zeolite component than a combination of theelectropositive metal component and the late transition metal compoundcomponent.
 4. The method of claim 1, wherein the gas comprises water oran alcohol contaminant.
 5. The method of claim 4, wherein the gascomprises a water contaminant.
 6. The method of claim 5, wherein thedecontaminant comprises about 70% by volume of an electropositive metalcomponent, about 15% by volume of a late transition metal component andabout 15% of a high silica zeolite component.
 7. The method of claim 1,wherein the gas comprises a nitrogen oxide, a sulfur oxide, an organicsulfide or an alkyl halide contaminant.
 8. The method of claim 7,wherein the gas comprises a sulfur oxide or a nitrogen oxidecontaminant.
 9. A method of decontaminating a gas, wherein said gascomprises one or more of alkaline, acidic polar, neutral non-polaraprotic and environmental gas contaminants, the method comprisingremoving said contaminants from the gas by passing the gas through abody of decontaminant comprising 10% to 80% by volume of aelectropositive metal component, 10% to 80% by volume of a high silicazeolite component and 10% to 80% by volume of a late transition metalcompound component.
 10. The method of claim 9, wherein the gas comprisesan amine contaminant, an acid contaminant or both.
 11. The method ofclaim 9, wherein the gas comprises a siloxane contaminant, a hydrocarboncontaminant or both.
 12. The method of claim 9, wherein thedecontaminant comprises a smaller proportion of electropositive metalcomponent than a combination of the high silica zeolite component andthe late transition metal compound component.
 13. The method of claim 9,wherein the decontaminant comprises a greater proportion ofelectropositive metal component than late transition metal component,and a greater proportion of high silica zeolite component than latetransition metal component.